CNT have excellent properties of electric characteristics, mechanical strength and others, and studies and developments thereof as ultimate new materials are being made energetically. CNT are produced in various methods of a laser vaporization method, an arc discharge method, a chemical vapor deposition method (CVD method), etc. At present, however, they are produced only as a mixture morphology of metallic CNT and semiconducting CNT in any production methods. In practical use, the properties of either one only of metallic or semiconducting CNT are needed in many cases, and therefore, the studies of separating and purifying metallic or semiconducting CNT alone from a CNT mixture are considered extremely important from the viewpoint of the practical use of CNT.
Heretofore, there are given some reports relating to separation of metallic CNT and semiconducting CNT from each other; however, there is known nothing applicable to large-scale production. For example, there are known a method of electrophoresing CNT dispersed with a surfactant, on a microelectrode (Non-Patent Reference 1); a method of using amines as a dispersant in a solvent (Non-Patent References 2, 3); and a method of selectively firing semiconducting CNT with hydrogen peroxide (Non-Patent Reference 4); however, these are problematic in that only metallic CNT could be obtained in all of them and the collection rate is low.
Also known is a method of separating CNT dispersed in a surfactant into metallic/semiconducting CNT through density-gradient ultracentrifugation (Non-Patent Reference 5); however, this requires an extremely expensive instrument of an ultracentrifuge and takes a long separation time of 12 hours, and has some other difficulties in that enlarging the instrument is limited and a large amount of samples could not be processed.
Further known is a method comprising dispersing CNT with a nonionic surfactant followed by applying a direct-current field to the solution to thereby make the metallic CNT move toward the cathode side while the semiconducting CNT are kept remaining on the anode side (Non-Patent References 6, 7). However, the method has some difficulties in that it has a necessity of using a nonionic surfactant of low dispersing ability, the collection rate is low, the separation time is 18 hours and is long, and it is difficult to accurately collect the separated product since the method is electrophoresis in a free solution.
Also known is a report of trying separation of metallic and semiconducting CNT from each other by controlling the pH and the ionic intensity of a CNT solution dispersed with a surfactant to cause a different degree of protonation depending on the type of CNT, followed by applying an electric field to the resulting solution for the intended separation (Patent Reference 1). However, the method requires a step of pretreating the suspended nanotube mixture with a strong acid for pH and ionic intensity control prior to separation, and therefore, severe process control for the step is inevitable and finally, in addition, the separation of metallic and semiconducting CNT from each other could not be attained.
Non-Patent Reference 1: Advanced Materials 18, (2006) 1468-1470
Non-Patent Reference 2: J. Am. Chem. Soc. 127, (2005) 10287-10290
Non-Patent Reference 3: J. Am. Chem. Soc. 128, (2006) 12239-12242
Non-Patent Reference 4: J. Phys. Chem. B 110, (2006) 25-29
Non-Patent Reference 5: Nature Nanotechnology 1, (2006) 60-65
Non-Patent Reference 6: Preprint for 32nd Fullerene Nanotube General Symposium Talk, p. 136, 3P-7 (Feb. 15, 2007, Meijo University, Aichi)
Non-Patent Reference 7: Preprint for 54th Applied Physics-Related Joint Lecture Presentation, Spring, 2007, No. 3, p. 1593, 27p-ZR-1 (Mar. 27, 2007, Aoyama Gakuin University, Kanagawa)
Patent Reference 1: JP-T 2005-527455